This invention relates to radio frequency modulation and in particular to a radio frequency modem having an oscillator switching circuit capable of switching the oscillator to operate on any one of a plurality of predetermined frequencies without switching crystals into or out of the oscillator circuit.
Many radio frequency oscillators operate on a single frequency having a crystal resonant with a tank circuit at the operating frequency. Radio frequency oscillators that operate on multiple frequencies typically switch the crystal and have a tank circuit with a wide response to oscillate with all of the relatively close crystal frequencies. The crystals are not left in the oscillator circuit and the crystal circuit only is switched because if the frequencies of the crystals are too close together the undersirable condition may occur that more than one crystal will resonate with the same tank circuit. When the crystal frequencies are spaced farther apart in the frequency spectrum such as a few megahertz, a single tank circuit cannot resonate with the relatively closely spaced crystal frequencies; it is then necessary to switch the tank circuits at least.
While these switching circuits have been adequate, switching crystals requires attention to the effect on frequency of the oscillator of the switching device. Where the frequencies of the oscillator are relatively far apart in the frequency spectrum, the single tank circuit will not suffice. It would be desirable to have a radio frequency oscillator having frequency switching capacility that requires no switching in the crystal circuit and provides a separate tank circuit for each frequency produced by the oscillator.